Several cuprate superconductors are already superconductive above the boiling point of liquid nitrogen (77° K.). These cuprate superconductors (called high-temperature superconductors (HTSL)), however, have poor mechanical properties. The development of band lines is an attempt to overcome the associated problems.
Band lines (also know as band-HTSL or band-shaped HTSL) are coated conductors including a superconductive functional layer applied to a band-shaped substrate via a special process. The functional layer may include, e.g., yttrium-barium-copper-oxide YBa2Cu3Ox (YBCO). As shown in FIG. 1, these band lines have a structure including a metal substrate, a buffer layer, and a superconductor layer. The economic efficiency of the production process is determined by the deposition process, as well as by the superconductive properties.
The main difficulty in the production of coated conductors is that the HTSL (superconductive) layer must have an extremely high degree of texture, that is, a high degree of crystallographic orientation. The individual crystallites of the layer should be tilted against one another only by a minimum value, since otherwise the superconducting properties are severely impaired.
To achieve such a high degree of texture, two different preparation processes may be utilized. It is common to both preparations that, before the superconducting layer is deposited, a textured buffer layer is produced and placed on the substrate. Thus, when the superconducting layer is deposited on the buffer layer, the texture (orientation) is transferred to the superconducting layer. In the two preparations, metal substrates are used, since this is the only way that the strength of the band lines necessary for later use in electrical technology can be achieved.
In the first preparation process, an untextured, crystallographically-non-oriented metal substrate formed from, e.g., Hastelloy® alloy is used. A textured buffer layer (i.e., a buffer layer with crystallographic orientation) is then applied to the untextured substrate. Such a direct deposition can be carried out only using physical coating processes under high vacuum (e.g., Ion Beam Assisted Deposition (IBAD) and Inclined Substrate Deposition (ISD)). Drawbacks of this process are high equipment costs (caused, for example, by the high vacuum pressure requirements) and a low deposition rate. In the second preparation, the metal substrate is already textured by special deformation and temperature treatment processes. The texture of the substrate can thus be transferred to the buffer layer and, in turn, to the superconducting layer deposited thereon. The advantage of this method is that no directed deposition processes must be used. Here, physical processes, such as Pulsed Laser Deposition (PLD) and Thermal Co-Evaporation (TCE) and chemical processes, such as Chemical Solution Deposition (CSD) and Metal-Organic Chemical Vapor Deposition (MOCVD) may be used. Again, the PLD and TCE processes require high vacuum pressure (and thus high equipment costs), even though they provide higher deposition rates than direct deposition processes.
Chemical coating processes (e.g., Chemical Solution Deposition (CSD)) are economical relative to physical coating processes since they work at normal pressure (i.e., without the need for high vacuum pressure), while providing a higher deposition rate. FIG. 2 shows two CSD processes. As shown, on the laboratory scale, coating with CSD processes may be carried out as a “dip-coating” process (FIG. 2A), in which the substrate is immersed into a solution and pulled back out, or as a “spin coating” process (FIG. 2B), wherein several drops of the solution are applied to a substrate and distributed by rotating the substrate (centrifugal force spreads the solution on the substrate). For production of greater lengths, the substrate band can be drawn through a coating solution and then dried in a furnace. A diagram of such a system can be seen in FIG. 3. As shown, the system includes a rinsing unit, a coating unit, a drying unit, and a winding unit. The subsequent reaction is carried out at a high temperature.
In the CSD process, the reaction to form an actual crystalline layer is carried out in several steps. First, the substrate is coated and then dried (i.e., the solvent is removed), producing an amorphous layer that consists of organic and/or inorganic metal salts. The decomposable portions of the salts are pyrolyzed in a subsequent step. The pyrolyzed layer is crystallized in a last annealing step to form the final layer. These partial steps can be performed both in succession and by means of a temperature program within a single annealing treatment.
The superconductive properties considered in evaluating the economy of a coated conductor or its production process are the critical current density (Jc) and the critical current (Ic). Typically, values at 77 K (boiling point of nitrogen) with a criterion of 1 μV/cm of voltage drop are taken as a basis. All values indicated later on for the critical superconductive properties relate to this temperature and this criterion. The texture of the superconductive layer is, as described above, responsible for the quality thereof and thus influences in particular the critical current density Jc. Current achievable, typical values for YBCO layers, which were deposited by means of CSD processes on various buffer layers, are current densities of about 2 MA/cm2. In this case, it is unimportant what approach was selected in accordance with the above-described design possibilities for coated conductors.
Possible buffer layer materials can be yttrium-stabilized zirconium oxide, gadolinium-zirconate, yttrium oxide, lanthanum-aluminate, lanthanum-zirconate, strontium-titanate, nickel oxide, cerium oxide, magnesium oxide, lanthanum-manganate, strontium-ruthenate and many others. In addition, layer combinations that consist of several different materials are possible. The maximum values for the critical current densities of CSD-YBCO layers are currently achieved in physically-produced buffer layers.
The critical current density is a central feature of a coated conductor, but this feature is not sufficient for the evaluation of the superconductive performance, since this value indicates nothing about the absolute critical current (Ic). A high, absolute critical current can only be achieved by comparatively thick, highly-structured YBCO layers, i.e., layers with high critical current density (Jc). Achieving a high degree of texture via the entire thickness of a superconductive layer is very difficult especially in layers that were deposited via a CSD process. While thin layers in the range of up to 50 nm have a very good texture and thus high Jc values, Jc is reduced when thicker layers are produced via conventional CSD processes, especially at the surface. Thus, it is not possible with the currently-used CSD process to produce coated conductors with a very high Ic.
One factor contributing to the inability to produce coated conductors with high Ic is the solvents and precursor systems currently used for the deposition of the YBCO layers by means of CSD. The above-cited favorable results relative to Jc were achieved utilizing a trifluoroacetate (TFA) system. In this coating system, all metals are dissolved as acetates in trifluoroacetic acid, such that formally complete metal-trifluoroacetates are produced, which remain on the substrate band in the drying of the coating solution after the evaporation of the acetic acid residues and trifluoroacetic acid. TFA solutions are primarily used since, when other, primarily organic metal salts are used, barium carbonate is produced during drying or in the pyrolysis of the coating. Barium carbonate is stable (it is not destroyed during the subsequent process conditions), and prevents the availability of barium for the formation of the YBCO superconductor, hampering current transport at the grain boundaries. Since barium fluoride is formed in the first pyrolysis step, the formation of barium carbonate in the TFA route is effectively prevented. The formation of barium fluoride also requires, however, that an annealing process must be added to the final crystallization of the superconductor water vapor—the water vapor must penetrate the deposited precursor layer from the surface to react with barium fluoride to form barium oxide and hydrofluoric acid. The hydrofluoric acid must, in turn, leave the layer. The required long diffusion paths for these relatively large molecules causes pores to form in the superconductor layer and, but prevents the growth of thick superconductive layers because, with increasing layer thickness, the diffusion possibilities are limited. This drawback can be partially avoided by a multi-layering. This approach, however, results in smaller and smaller increases in layer thickness and poorer textures in the case of the increasing number of coating cycles.
In addition to the diffusion problem, the use of fluorinated precursors in the coating solution is problematic because of the toxicity of the hydrofluoric acid that is produced during the processes. Hydrofluoric acid or hydrogen fluoride is already classified as very toxic even in the smallest concentrations. In technical systems, a large number of monitoring measures must be taken to prevent employee exposure and emissions into the environment. Moreover, the amount of trifluoroacetic acid (TFA) that is used should be kept small because of its ecotoxic action as well as biological effects (damaging action on water organisms can have long-lasting harmful effects in bodies of water, damaging action by pH shift, still caustic even in dilution, danger for drinking water). It is therefore desirable, both for reasons of safety at work and for environmental protection, as well as for reasons of process efficiency, to drop the fluorine content of the coating solution as much as possible or to dispense with fluorine-containing precursors completely.
Previously, no fluorine-free coating process for the superconductive layer could be developed for the production of YBCO layers, which also only approximately have the superconductive properties such as layers that were produced by means of the TFA method. Obviously, the use of fluorine-free systems is not possible because of the above-described problems of the barium carbonate formation. Consequently, focus has been placed on reducing the fluorine contents of the coating systems. This is carried out by, e.g., utilizing a mixture that consists of trifluoroacetic acid and acetic acid as a solvent. Thus, as in a pure TFA process, not all metal ions are formally completely present as trifluoroacetate but rather only, e.g., the barium ions. The other ions, i.e., copper and yttrium, are present in these systems as acetates or oxides after the coating solution is dried. The coating systems that are reduced in the fluorine portion reduce the release of hydrofluoric acid or hydrogen fluoride, and thus the environmental costs in the process. Since, however, the diffusion paths through the entire layer thickness are very long, no significant improvements in the texture or current density of thicker superconductive layers could be achieved with these systems.